Variable-speed wind power system with improved energy capture via multilevel conversion

ABSTRACT

A system and method for efficiently capturing electrical energy from a variable-speed generator are disclosed. The system includes a matrix converter using full-bridge, multilevel switch cells, in which semiconductor devices are clamped to a known constant DC voltage of a capacitor. The multilevel matrix converter is capable of generating multilevel voltage wave waveform of arbitrary magnitude and frequencies. The matrix converter can be controlled by using space vector modulation.

CROSS REFERENCE TO RELATED APPLICATION

[0001] The present application claims priority to U.S. ProvisionalApplication No. 60/384,637, filed May 31, 2002. Said ProvisionalApplication is incorporated herein by reference.

CONTRACTUAL ORIGIN OF THE INVENTION

[0002] The United States Government has rights in this invention underContract No. DE-AC36-99GO10337 between the United States Department ofEnergy and the National Renewable Energy Laboratory, a division of theMidwest Research Institute.

BACKGROUND OF THE INVENTION

[0003] Power converters are used to convert alternating current (AC)electric power from a fixed-frequency and fixed-voltage to differentfrequencies and different voltages for powering loads, such as electricmotors. They are also used in reverse to convert variable-frequency,variable-voltage, AC electric power to fixed-frequency, fixed-voltageelectric power. Electric generators driven by wind turbines rotate atdifferent speeds, depending on wind conditions, so they producevariable-frequency, variable-voltage, AC electric power. Therefore,power converters are used to convert such wind-generated electric powerto fixed-frequency, fixed-voltage, AC power to match public utility andsimilar AC power systems. However, wind turbine power systems also spendlots of time operating at light loads or fractions of their rated powercapacities, whereas standard, state-of-the-art, power converters aredesigned to operate most efficiently at full-rated power all, or nearlyall, of the time. Further, standard power converters do not work at lowvoltages. Therefore, when wind turbine-driven generators are operatingin low wind, light load, conditions, standard power converters areinefficient and may not work at all.

SUMMARY OF THE INVENTION

[0004] It is a general object of this invention, therefore, to provideimprovements over existing variable-speed, wind power technology thatachieves high efficiency at low wind speeds and captures more windenergy.

[0005] Another object of the invention is to provide improvements in ACpower converter technologies to enable more efficient and reliableconversion of variable-frequency, variable-voltage, AC power tofixed-frequency, fixed-voltage, AC power and vice versa.

[0006] A more specific object of this invention is to provide a moreefficient and reliable power converter for wind power generatorapplications in which generated AC power varies over wide ranges offrequencies, voltages, and load levels, for converting such AC power tofixed-frequency, fixed-voltage, AC power for public utility and similarAC power systems.

[0007] Additional objects, advantages, and novel features of theinvention shall be set forth in the description that follows, and otherswill become apparent to persons skilled in the art upon examination ofthe following or may be learned by the practice of the invention. Theobjects and advantages may be realized and attained by means of theinstrumentalities and combinations particularly pointed out in theappended claims.

[0008] To achieve the foregoing and other objects and in accordance withthe purposes of the present invention, as embodied and described eitherin general principles, in details, or both, the power convertercomprises any one, or combinations of, the following features: (i)Multilevel power conversion is realized using a dc-link converter systemin which at least the rectifier side operates with voltages switchingbetween more than two levels and the switch modulation strategy iscontrolled as necessary to optimize efficiency at each intended turbinespeed; (ii) Multilevel power conversion is realized using a multilevelmatrix converter, and the switch modulation strategy is controlled asnecessary to optimize efficiency at each intended turbine speed; and(iii) Several multilevel converters may be connected in parallel and, byselectively disabling one or more of these multilevel converters at lowwind speed conditions, the fixed losses are reduced.

[0009] Variable-speed wind powered generators attempt to captureadditional energy from the wind by optimizing turbine speed over a rangeof wind speeds. A problem with this approach is that it is difficult tooptimize the electric power conversion parts of the system to operatewith high efficiency at all turbine speeds. In particular, as thegenerator voltage is reduced, the efficiency of the power converter istypically degraded. A significant part of this invention is thediscovery that light load inefficiencies of ordinary power convertersused in wind power generation applications, such as fixed losses withinthe converters operating at reduced or low power levels, are moresignificant than was understood previously by persons skilled in theart, as well as the further discovery that the modularity principlediscussed above is a very effective way to reduce such light loadinefficiencies by operating an optimum number of smaller converters ator near their optimum efficiency levels, depending on wind speed andload conditions, instead of operating one larger power converter atinefficient voltage or power levels. In other words, any given total ACpower output of the wind turbine generator can be handled by a pluralityof smaller power converters, each one operating at or near its optimumhigh power converter efficiency level. The number of such smaller powerconverters actuated and operating at any given time is the number ofthem required to handle the particular total AC power load that existsat that time. Multilevel conversion is also applied, with voltagesswitching between more than two levels, whereas conventional powerconverters have a large switch that operates rapidly, switching betweentwo positive and negative voltage levels or peaks, e.g., −700 volts to+700 volts. The multilevel switching algorithm is altered when windgenerator voltage is low in magnitude to improve converter efficiency. Anew multilevel matrix converter system is used to implement suchmodularity in the multilevel voltage ranges. Inductance is used insteadof capacitance, which accommodates use of a buck-boost voltage tosustain converter operation in low wind, low voltage conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Other objects and advantages of the invention will becomeapparent upon reading the following detailed description and uponreference to the drawings in which:

[0011]FIG. 1 is a schematic diagram of a basic matrix converteraccording to one aspect of the invention;

[0012]FIG. 2 is a schematic diagram of one of the switch cells of thematrix converter shown in FIG. 1;

[0013]FIG. 3 is a schematic diagram of three possible switchingcombinations for one choice of branch connection;

[0014]FIG. 4 is a schematic diagram of the coordinate system used torepresent space vectors, which facilitates illustration and explanationof some aspects of the invention;

[0015]FIG. 5 illustrates the space vectors attainable by the basicmatrix converter shown in FIG. 1;

[0016]FIG. 6 is a diagram illustrating the synthesis of a referencevoltage from the adjacent space vectors;

[0017]FIG. 7 shows an example simulated utility-side space vectormodulated (“SVM”) line-line voltages and inductor currents, with a unitypower factor, for the basic matrix converter shown in FIG. 1;

[0018]FIG. 8 shows the harmonic spectrum of the utility-side voltage ofthe example in FIG. 7;

[0019]FIG. 9 shows a simulated generator-side space vector modulated(“SVM”) line-line voltages and inductor currents;

[0020]FIG. 10 shows the harmonic spectrum of the utility-side voltage atan example increased switching frequency of 20 kHz;

[0021]FIG. 11 shows an example simulated utility-side voltage andinductor currents, with a non-unity power factor;

[0022]FIG. 12 is a schematic diagram of a multilevel matrix converterhaving multiple switch cells in series in each branch connecting aninput phase to an output phase;

[0023]FIG. 13 is a function block diagram of a control system for amultilevel matrix converter according to another aspect of theinvention;

[0024]FIG. 14(a) is a diagram illustrating the switch combinations forobtaining an input reference voltage space vector between V₃ and V₄, andan output reference voltage space vector 180° from the input vector andbetween V₆ and V₁; and

[0025]FIG. 14(b) is a voltage waveforms resulting from the combinationshown in FIG. 14(a).

[0026] While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EXAMPLE EMBODIMENTS

[0027] Multilevel Matrix Converter

[0028] Referring to FIG. 1, a three-phase multilevel matrix converter100 in one example embodiment of the invention is illustrated as beingconnected at the input end 102 to a three-phase ac generator 110 and atthe output end 104 to a three-phase utility system 120. The converter100 includes nine switch cells 130, each of which links together someunique combination of an input phase (a, b, or c) and output phase (A,B, or C). Conversely, each combination of input and output phases islinked by a unique switching element, as will be described below. Eachof such individual switching elements is identified for convenience bythe combination of phases that it links together. Thus, for example, theparticular switch cell 130 that is connected between the input phase aand the output phase A considered to be switch cell 130-aA, theparticular switch cell 130 that is connected between the input phase band the output phase, and so on.

[0029] The generator 110 can be any electrical generator. For example,it can be a wind turbine generator or a motor.

[0030] The matrix converter 100 further includes filter inductors 140-a,-b, and -c at the generator side, or input end 102, and 150-A, -B and -Cat the utility side, or output end 104. Any suitable inductors can beused. In an illustrative embodiment, all six inductors are the same andhave the following characteristics: Inductance: 0.2 mH Saturationcurrent: 50 A Winding: 26 turns of 8 ounce copper foil Air gap: 1.64mmCore material: iron-based METGLAS amorphous alloy cores, Bmax = 1.4Tesla Core size: AMCC 63 Dimensions: H = 10.2 cm W = 5.2 cm L = 3.0 cmCore cross-sectional area: 3.9 cm²

[0031] Each of the switch cells 130-aA, -aB, -aC, -bA, -bB, -bC, -cA,-cB and cC in the illustrative embodiment has an H-bridge network asillustrated in FIG. 2. Each cell 130 has four identical units, each ofwhich has a diode 210 and a transistor 220. The gate 222 of thetransistor 220 is to be connected to a controller (not shown in FIG. 2),which turns the transistor 220 on or off. The transistor 220 and thediode 210 are connected together in an anti-parallel fashion. Anysuitable power transistor and diode can be used. In particular,isolated-gate bipolar transistors (“IGBT”) are well suited for powerswitching applications. In an illustrative embodiment, the 600 V “SMPSSeries” n-channel IGBT devices (Intersil part # HGTG30N60A4D) were used.The TO-247 packages include anti-parallel “hyperfast” diodes. Thedatasheet ratings are:

[0032] Collector-to-emitter voltage 600 V;

[0033] Continuous collector current 60 A; and

[0034] Pulsed collector current 240 A.

[0035] The four transistor-diode pairs are connected to form afull-bridge inverter, with a positive node 240 and negative node 250. ADC bus capacitor 230 is connected between the positive and negativenodes 240, 250. The transistors and diodes within each cell are clampedto the capacitor voltage, V, which can be regulated to a known value.Thus, each switch cell 130 is capable of producing between the input andoutput terminals (a-A, a-B, etc.) the instantaneous voltages +V, 0, and−V, when at least one of the transistors 220 conducts, and is capable ofblocking voltages of magnitude less than V when all of the transistors220 are off.

[0036] The use of four transistors in the switch cell 130, as shown inFIG. 2 allows the average current to be doubled, relative to aconventional matrix converter whose four-quadrant switches are realizedusing two transistors and two diodes. This result is obtained, becausethe currents conducted by the IGBTs are thermally limited, and, byproper control, the current stresses can be spread over all fourtransistors 220 in FIG. 2.

[0037] The circuit of FIG. 1 is capable of limited multilevel operation.The semiconductor devices must be rated at least as large as the peakapplied line-to-line voltage. The converter 100 is capable of bothincrease and decrease of the ac voltage magnitude.

[0038] The number of voltage levels can be increased. FIG. 12illustrates one approach to increasing the terminal voltage levels. Theswitch cells, for example cells 130-aA-1 through 130-aA-n (total of ncells) are connected in series in each branch of the switch matrix. Thisallows increase of the terminal voltages without changing the voltageratings of the semiconductor devices. Multiple switch cells or stringsof serially connected switch sells can also be connected in parallel ineach branch, preferably with an inductor in series with each cell ofstring of cells as a current isolator.

[0039] The multilevel matrix converter 100 according to the inventionsynthesizes the input and output voltage waveforms by switching theknown capacitor voltages of the switch cells 130. This operation differsfrom that of conventional matrix converters in which voltage waveformsare synthesized on one side and current waveforms on the other. Becauseof the symmetry of the converter 100, both step-up and step-down of thevoltages are possible.

[0040] Although all nine switch cells 130 can be conducting at any giventime, in certain cases it may be desirable that fewer cells beconducting at once. For example, because of the inductors 140 and 150 atboth sides of the converter 100, current typically should flowcontinuously through the input and output phases. Hence, operation ofthe switch cells 130 should typically not lead to the open-circuiting ofan input or output phase. Further, conduction of the switch cells 130should typically not form a closed loop within the branches of theswitch matrix, since such a closed loop could short-circuit thecapacitors 230 of the switch cells. Third, the voltage applied to anopen switch cell 130 should not exceed the magnitude of its capacitor230 voltage. These constraints limit the possible connections within theswitch matrix and imply that, at any given instant under normaloperating conditions, exactly five of the nine branches of the switchmatrix should conduct. Further, the following rules typically apply tothe connections that are possible at a given instant:

[0041] There is exactly one connection path between any two phases;

[0042] If any phase on one side (i.e., the input side 102 or output side104) is connected directly to two conducting branches, then there mustbe exactly one other phase from the same side also connected directly totwo conducting branches. The third phase must be connected directly toone conducting branch;

[0043] If any phase on one side (i.e., the input or output side) isconnected directly to three conducting branches, then the other twophases from the same side must be each connected directly to exactly oneconducting branch;

[0044] Table I summarizes the possible configurations. There are a totalof 81 valid choices of branch connections. TABLE I POSSIBLE BRANCHCONNECTION CONFIGURATIONS Phase A or a Phase B or b Phase C or c 1branch 1 branch 3 branches 1 branch 2 branches 2 branches 1 branch 3branches 1 branch 2 branches 1 branch 2 branches 2 branches 2 branches 1branch 3 branches 1 branch 1 branch

[0045] The converter 100 can interface two asynchronous three-phase acsystems. Both interfaces are inductive in nature, whether intrinsicallyor through addition of series inductors 140, 150. In the configurationof FIG. 1 the converter 100 has nine branches that each has a switchcell 130 as shown in FIG. 2. As noted above, to avoid interrupting thesix inductor currents, exactly five branches typically must conductcurrent at any instant in time. It is also important to avoid thecross-conduction and shoot-through currents that can occur when thetransistors 220 of six or more branches conduct. However, turning offthe transistors of five or more branches does not cause a calamity,because the antiparallel diodes 210 can conduct current and provide apath for the inductor currents to flow. Energy stored in the inductors140, 150 is then transferred to the capacitors 230 of the switch cells130. One simple method for controlling the switching transitions is tofirst turn off all transistors 220 that are to be switched off, and thenafter a short delay, turn on the transistors 220 that are to be switchedon. Myriad other soft-switching schemes can also be used in thisinvention. Such other soft-switching schemes are known and can easily beapplied by persons skilled in the art, once he or she understands theprinciples of this invention, and thus need not be described further.

[0046] Each switch cell 130 of the multilevel matrix converter 100 hasthree switch states corresponding to voltages of +V, 0, and −V_(x),which means that there are three switch states that a switch cell 130may assume when it is used in a conducting branch. Since there are m(e.g., five) branches that may be turned on at any particular instantand three switch states per conducting switch cell 130, the number ofpossible device switching combinations for each case of branchconnection is 3^(m) possible device switching combinations. With 81cases of branch connections, the total number of device switchingcombination for five branches becomes 3⁵×81=19683 possible deviceswitching combinations.

[0047]FIG. 3 shows an example of three different device switchingcombinations for one case of a branch connection with branchesconducting between phases A-a, B-a, C-a, C-b, and C-c. FIG. 3 shows thatit is possible to obtain five different output voltage levels from themultilevel matrix converter 100 by switching only the devices of oneswitch cell 130 (in branch C-c). For this example, if it is assumed thatthe midpoint capacitor voltage V is set to +240 V_(x), FIG. 5(a)produces 0 volts for all line-to-line voltages on both sides of theconverter 100. This result can be obtained by operating all switch cells130 of the conducting branches to produce voltages of +240 V. Bychanging the switch cell 130-cC connecting branches C-c to produce avoltage of zero, the converter 130 can produce line-to-line outputvoltages of −240 V, 0 V, and +240 V, as shown in FIG. 5(b). In FIG.5(c), output line-to-line voltages of −480 V, 0 V, and +480 V areobtained. By alternating between the three device switching combinationsof FIG. 5, the basic multilevel matrix converter 100 can producefive-level voltage waveforms with voltage levels at −480 V, −240 V, 0,240 V, and 480 V at one side of the matrix converter 100. In each case,the nonconducting switch cells 130 block voltages of magnitude 240 V.

[0048] Control

[0049] The controller of the multilevel matrix converter typically mustperform the following major tasks.

[0050] 1. Maintain fixed voltage (charge regulation) across all midpointcapacitors 230; and

[0051] 2. Synthesize input and output voltage waveforms.

[0052] According another aspect of the invention, control thatsimultaneously handles the above two tasks is accomplished by spacevector modulation (“SVM”). In visualizing space vector modulation,vectors in a two-dimensional coordinate system can be used to representa three-phase voltage, because the three phases are not independent.Referring to FIG. 4, such coordinate system can be defined relative tothe stator 410 of a two-pole salient-rotor synchronous machine 400. The“direct” axis (“d-axis”) is the axis along which the gap 430 between thestator 410 and the rotor 420 is the smallest.

[0053] Upon analysis of all possible switching combinations, it is foundthat the nineteen space vectors illustrated in FIG. 5 (by solid rounddots) are attainable at each side of the converter. Control of theinput-side voltage is achieved by modulating between space vectorsadjacent the desired reference vector. Simultaneously, similar controlis applied to control the output-side voltage. Even when both the inputand output-side voltages are controlled, there exist additional degreesof freedom that can be used to control the capacitor voltages V.

[0054] For example, consider the space vector modulation illustrated inFIG. 6. At a given point in time, it is desired to produce the referencespace vector V_(REF), which can be accomplished by modulating betweenthree adjacent space vectors V₀, V₁, and V₂. The reference space vectorV_(REF) is expressed as a linear combination of the space vectors V₀,V₁, and V₂:

V _(REF) =d ₀ V ₀ +d ₁ V ₁ +d ₂ V ₂  (1).

[0055] The duty cycles d₁, d₂, and do represent the durations for deviceswitching combinations producing the space vectors V₀, V₁, and V₂,relative to the space vector modulation period. Since only three spacevectors are used in this example, the three duty cycles must add tounity. The duty cycles are found by solution of the geometry of FIG. 7:$\begin{matrix}\begin{matrix}{d_{1} = {{V_{1}} = {{V_{REF}}{\sin (\varphi)}}}} \\{d_{1} = {{\frac{V_{REF}}{V_{1}}{\sin (\varphi)}} = {M\quad \sin \quad (\varphi)}}} \\{{d_{2}{V_{2}}} = {{V_{REF}}{\sin \left( {60^{{^\circ}} - \varphi} \right)}}} \\{d_{2} = {{\frac{V_{REF}}{V_{2}}{\sin \left( {60^{{^\circ}} - \varphi} \right)}} = {M\quad \sin \quad \left( {60^{{^\circ}} - \varphi} \right)}}} \\{d_{0} = {1 - d_{1} - d_{2}}} \\{M = {\frac{V_{REF}}{V_{1}} = \frac{V_{REF}}{V_{2}}}}\end{matrix} & (2)\end{matrix}$

[0056] The term M in (2) is the modulation index, and its value cannotexceed unity as long as |V_(REF)|≦|V₁| and |V_(REF)|≦|V₂|. Thus, thereference space vector V_(REF) is synthesized by modulating throughswitch 130 configurations producing the space vectors V₀, V₁, and V₂during a given SVM period.

[0057] With the above approach, the multilevel matrix converter 100 iscapable of operating with universal input and output voltage, frequency,and power factor.

[0058] To illustrate operation of the proposed multilevel matrixconverter, simulations of operation at two different operating pointsare given in FIGS. 7 to 12. In these examples, the multilevel matrixconverter interfaces a variable-speed wind generator to a 60 Hz utility.FIG. 7 shows simulated voltage and current waveforms for the three-phaseac utility side. The utility side is at 240 V, 11 A, 60 Hz, and unitypower factor. For this operating point, the generator side is at 240 V,25 Hz, and unity power factor. The converter 100 switching frequency isset to 1 kHz. The simulator implements the space vector modulationdescribed above to control the multilevel matrix converter 100, andhence synthesizes the desired pulse-width modulation (“PWM”) input andoutput voltage waveforms. The pulse-width modulated waveforms in FIG. 7are line-to-line voltages and the sinusoids are phase currents. Sincethis phase sequence is positive, the set of phase currents lags the setof line-to-line voltages.

[0059]FIG. 8 shows the harmonic spectrum of utility side line-to-linevoltage V_(A B). Notice the high magnitude harmonics in the vicinity ofthe 18th harmonic, which are due to the switching frequency of 1 kHz.

[0060]FIG. 9 shows the generator-side voltage and current waveforms forthe same operating point. The simulator is programmed to selectappropriate device switching combinations to maintain constant thevoltages across all switch cell capacitors 230 in addition tosynthesizing the desired input and output waveforms. Using nine switchcells 130 constitutes the most basic converter 100 configuration withone switch cell 130 per branch. All capacitor voltages are maintainedwithin +/−12% of their nominal values.

[0061]FIG. 10 illustrates the effect on the spectrum of increasing theswitching frequency to 20 kHz. The harmonic spectrum of the utility sideline-to-line voltage V_(AB) is plotted. Notice that all of the highmagnitude harmonics of FIG. 10 are moved to higher harmonic numbers,corresponding to the 20 kHz switching frequency.

[0062] Operation at non-unity power factor is illustrated next. At thispoint, the utility-side 104 voltage and current are 240 V 11 A, 60 Hz,0.5 power factor, with 1 kHz switching frequency. The generator side 102operates at 60 V, 6.25 Hz, and unity power factor. The utility-side 104waveforms are illustrated in FIG. 11.

[0063] Thus, operation of the multilevel matrix converter 100 accordingto the invention is confirmed. The utility-side 104 and generator-side102 waveforms can be controlled simultaneously and independently. Thecapacitor 230 voltages of the switch cells 130 can also be regulated.

[0064] Referring to FIG. 13, a control system and its operationaccording to another aspect of the invention are explained. The utilityand generator voltages and currents are sensed by sensing circuits 1302such as Hall effect devices, and are digitized using analog-to-digitalconverters (ADC) 1306. The microcontroller 1304 transforms these intod-q coordinates. The dc capacitor 230 voltages of the nine switch cellsare 130 also measured using differential amplifier circuits, anddigitized using ADC's. The microcontroller 1304 performs the spacevector modulation algorithm described above, and commands the switchingof the semiconductor devices. The microcontroller is interfaced to theswitch cells through complex programmable logic devices (CPLDs); in theillustrative system, there are a total of five CPLD's (timer 1308, databuffer 1312, and switch cell control 1320-a, -b and -c) and three flashmemory chips (lookup table chips 1316-a, -b and-c). The CPLDs areaddressable by the microcontroller 1304, and store the current state ofall switches. In addition, the state of all switches during the nextsub-interval can be loaded into the CPLDs. At the beginning of asubinterval, the microcontroller 1304 commands the CPLDs to change theiroutputs to the new states, thereby causing the IGBTs 220 to switch.

[0065] To avoid cross-conduction of the IGBTs 220 during their switchingtransitions (which would lead to momentary shorting of the dc busvoltages through the IGBTs), the turn-off transitions of the IGBTsoccurs first. In other words, those IGBTs that were previously on, butwill be turned off, are switched first. After a controllable delay (the200 ns block 1318 illustrated in FIG. 13), the turn-on transitions aretriggered (i.e., the IGBTs that were previously off, but will be turnedon, are switched). The outputs of the CPLDs are connected throughisolated gate driver chips (not shown) to the IGBTs.

[0066] As an example, suppose (arbitrarily) that an input referencevoltage space vector between V₃ and V₄, and an output reference voltagespace vector 180° from the input vector (between V₆ and V₁) is desired.FIG. 14(a) illustrats the switch combinations for obtaining such aresult, and FIG. 14(b) is a voltage waveforms resulting from thecombination shown in FIG. 14(a).

[0067] The particular embodiments disclosed above are illustrative only,as the invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular embodiments disclosed above may be altered or modified andall such variations are considered within the scope and spirit of theinvention. Accordingly, the protection sought herein is as set forth inthe claims below.

What is claimed is:
 1. A matrix converter, comprising: a plurality ofinput ends; a plurality of output ends; and a plurality of multilevelswitch cells, each one of the plurality of input ends being electricallylinked to each one of the plurality of output ends via at least one ofthe plurality of multilevel switch cells.
 2. The matrix converter ofclaim 1, further comprising a plurality of inductors, wherein each ofthe plurality of input and output ends is electrically connected to atleast one of the plurality of inductors.
 3. The matrix converter ofclaim 1, wherein each of the plurality of multilevel switch cellscomprises: a full bridge inverter, comprising a first and secondterminals, a positive and negative nodes, and four inverter arms, eacharm comprising a diode having a positive pole and a negative pole, and atransistor having a gate, an emitter and collector, with the emitterelectrically connected to the negative pole and the collectorelectrically connected to the positive pole, a first and second of thefour inverter arms being electrically connected to the positive node tothe first and second terminal, respectively, and a third and fourth ofthe four inverter arms being electrically connected to the negative nodeto the first and second terminal, respectively, with the diode in eachof the four arms forwardly directed toward the positive node; and acapacitor electrically connected between the positive and negativenodes.
 4. The matrix converter of claim 3, further comprising aplurality of inductors, wherein each of the plurality of input andoutput ends is electrically connected to at least one of the pluralityof inductors.
 5. The matrix converter of claim 1, wherein each one ofthe plurality of input ends being electrically linked to each one of theplurality of output ends via at least two of the plurality of multilevelswitch cells.
 6. The matrix converter of claim 5, wherein the at leasttwo switch cells are electrically connected in series with each other.7. The matrix converter of claim 5, wherein the at least two switchcells are electrically connected in parallel with each other.
 8. Thematrix converter of claim 1, further comprising a controllerelectrically connected to the switch cells and adapted to change theelectrical potential difference between at least one of the plurality ofoutput ends and at least one of the plurality of input ends.
 9. Thematrix converter of claim 3, further comprising a controllerelectrically connected to the gates of the transistors adapted to turnthe transistors off and on.
 10. The matrix converter of claim 8, whereinthe controller is a pulse width modulator.
 11. An electrical powergenerator, comprising: an energy conversion unit having a plurality ofoutput ends and adapted to convert non-electrical energy intomulti-phase ac electricity and to supply each phase of the electricityto a respective one of the plurality of output ends; and a matrixconverter of claim 1, at least a subset of the plurality of the inputends of the matrix converter being electrically connected to the outputends of the energy conversion unit.
 12. The generator of claim 11,wherein the energy conversion unit is a variable-speed wind turbine. 13.A method of synthesizing a voltage, the method comprising: providing aninput multi-phase ac electrical voltage; applying each of the input acelectrical voltage to a respective one of a plurality of input ends of amatrix converter, which comprises: a plurality of output ends, and aplurality of multilevel switch cells, each one of the plurality of inputends being electrically linked to each one of the plurality of outputends via at least one of the plurality of multilevel switch cells; andcontrolling the voltages at the plurality of output ends.
 14. The methodof claim 13, wherein each of the plurality of multilevel switch cellscomprises a full bridge inverter, comprising: a first and secondterminals, a positive and negative nodes, and four inverter arms, eacharm comprising a diode having a positive pole and a negative pole, and atransistor having a gate, an emitter and collector, with the emitterelectrically connected to the negative pole and the collectorelectrically connected to the positive pole, a first and second of thefour inverter arms being electrically connected to the positive node tothe first and second terminal, respectively, and a third and fourth ofthe four inverter arms being electrically connected to the negative nodeto the first and second terminal, respectively, with the diode in eachof the four arms forwardly directed toward the positive node; and acapacitor electrically connected between the positive and negative nodesand the step of changing the electrical potential difference compriseschanging the on/off state of at least one of the transistors.
 15. Themethod of claim 13, wherein the step of controlling the voltagescomprises modulating the voltages at an output end of the matrixconverter by pulse-width modulation.
 16. The method of claim 15, whereinthe modulating step comprises space vector modulation.
 17. The method ofclaim 13, wherein each one of the plurality of input ends beingelectrically linked to each one of the plurality of output ends via atleast two of the plurality of multilevel switch cells in parallel witheach other, and further comprising turning off one of the at least twoswitch cells while keeping the other of the two on.
 18. The method ofclaim 17, wherein the step of apply the input voltages compriseselectrically connecting the matrix converter to a wind turbine electricgenerator, and turning off one of the at least two switch cells whilekeeping the other of the two on when the wind turbine rotate at a speedbelow a predetermined value.
 19. An electrical system, comprising: afirst multi-phase ac system having a first plurality of terminals, eachof which is adapted to conduct electricity of one of a first pluralityof phases to or from the first multi-phase ac system; a secondmulti-phase ac system having a second plurality of terminals, each ofwhich is adapted to conduct electricity of one of a second plurality ofphases to or from the second multi-phase ac system; and a plurality ofmultilevel matrix converters, each of which comprising: a plurality ofinput ends, a plurality of output ends, and a plurality of multilevelswitch cells, each one of the plurality of input ends being electricallylinked to each one of the plurality of output ends via at least one ofthe plurality of multilevel switch cells, at least a subset of theplurality of the input ends of each matrix converter being electricallyconnected to the first plurality of terminals of the first multi-phaseac system, and at least a subset of the plurality of the output ends ofeach matrix converter being electrically connected to the secondplurality of terminals of the first multi-phase ac system.
 20. Theelectrical system of claim 20, wherein the first ac system comprises avariable-speed wind turbine generator, and the second ac systemcomprises a utility grid.